(i) Field of the Invention
The present invention relates to a phosphate treatment solution for treating the surfaces of automobile bodies constituted by the combination of steel plates and other materials such as zinc and aluminum, i.e., the composite structures, and it also relates to a method for the treatment.
(ii) Description of the Prior Art
Heretofore, when an automobile body containing aluminum materials as constitutional parts is treated with a phosphate solution, the aluminum parts are first subjected to a chromate treatment mainly to heighten performance, and the thus treated aluminum parts are then assembled to the automobile body comprising steel plates and zinc-plated steel plates. Afterward, the phosphate treatment is carried out again, followed by a cathodic electrodeposition coating. According to this known technique, chromium and aluminum are partly dissolved out from the first formed chromate coating on the aluminum parts in the subsequent phosphate treatment step, so that the chromate coating tends to become in an imperfect state, and thus it is natural that the phosphate coating is not formed, either.
In the above-mentioned technique, the aluminum parts are subjected to the chromate treatment prior to assembling these aluminum parts to the automobile as described above, and therefore chromium and aluminum are dissolved out in the subsequent phosphate treatment step, so that the chromate coating and the phosphate coating becomes in an imperfect state. In consequence, when a paint coating is subsequently carried out, the resulting paint film is poor in adhesive performance, and in particular, there is a problem that the secondary adhesion after water-soaking (hereinafter, wet adhesion) is poor.
In the manufacturing process of the automobile bodies, parts assembly, pretreatment and paint coating are carried out in this order, and in the conventional process, the aluminum parts are separately treated by another procedure. That is, the aluminum parts are subjected to a water-rinsing, a chromate treatment and a water-rinsing/drying in this order, and further subjected to the above-mentioned assembly, the pretreatment and the paint coating. Therefore, there is also the problem that operating efficiency is bad and cost is high.
In the case that the aluminum parts are assembled to the automobile body without performing any chromate treatment and then subjected to the phosphate treatment, the conventional known treatment solution cannot provide any phosphate coating having satisfactory performance, i.e., excellent filiform corrosion resistance and wet adhesion, on the surfaces of the aluminum parts. A poor coating is merely formed which is unsuitable for the automobile bodies where the high paint film performance is required. In addition, aluminum ions are dissolved into the phosphate treatment solution in this treatment step, and inconveniently, these aluminum ions have a bad influence on the phosphate coating on the surfaces of other kinds of materials in the automobile body.